Why The Fat You Can See Isn’t The Fat You Should Worry About

If you have a healthy BMI, you don't have to worry about your weight, right?

Not necessarily.

In reality, the area of your body where you store your fat may be a better predictor of health -- regardless of your body mass index.

"All fat is not the same," said Dr. Virend Somers, a cardiologist at the Mayo Clinic in Rochester, Minnesota. Fat directly under the skin -- the stuff we can see -- isn't necessarily harmful.

It's the invisible fat that we need to be concerned about.

"The fat inside is the one that we’re realizing is harmful," Somer said. "It’s much more metabolically active and produces all kinds of bad things. It’s the kind of fat that’s linked to high cardiovascular risk."

The difference between visible and invisible fat

Subcutaneous fat is the visible kind that we're used to seeing, the stuff that you can pinch on your stomach or thighs. This type of fat lives under the skin, and is relatively safe, health-wise. Pear-shaped people, who have smaller waists and store weight in their hips, also tend to have more subcutaneous fat.

Apple-shaped people, who store weight around their mid-sections, tend to have more visceral fat. Visceral fat is internal, and you can't usually tell if someone has it just by looking. Visceral fat collects around organs such as the stomach, intestine, spleen and kidneys.

"You don’t see this fat early on," Somers explained. If you're lean and you start putting on weight, without seeing any visible outward changes, that could be visceral fat. "Then suddenly it starts bulging out and that’s when the belly starts hanging out," he said.

A small 2010 study compared two groups of young, healthy-weight participants. One group maintained their weight over eight weeks, but researchers overfed the second group, causing them to gain about 10 pounds each on average. The health results, published in the Journal of the American College of Cardiology, were striking.

Among the fat-gain group, some participants gained more visceral fat and some gained more subcutaneous fat. Those who gained the most visceral fat, about a third of participants, saw the biggest deterioration in blood pressure and in endothelial function, a measure of how well the blood vessels are working. At the end of the 16 weeks of the study, when the visceral fat-gainers had lost the excess weight, their endothelial function returned to normal.

Those who gained mostly subcutaneous fat didn't see any significant change in endothelial function.

"They didn’t look that different," Somers, one of authors of the study, said of the participants' modest weight gain. "It’s not merely a function of getting fat. It’s where the fat goes."

How do I know if I have visceral fat?

Unfortunately, the body-fat scale at your gym, which uses a method called bioelectrical impedance analysis, isn't especially accurate. The gold standards for visceral fat analysis include MRIs, CT scans and hydrostatic water weighing, which don't come cheap. If you're not interested in investing in a pricey test or two, a good rough proxy is to calculate your waist-to-hip ratio. For men, the ratio should be no higher than 0.90, for women, no higher than 0.83.

Don't despair if the resulting number isn't what you'd hoped -- it's easy enough to start addressing the problem, because "Exercise disproportionately targets visceral fat,” Gary R. Hunter, a professor of human studies at the University of Alabama at Birmingham, told the New York Times in 2015.

Hunter's research bears this out. A 2010 study published in the journal Obesity found that sedentary women who started a twice-weekly, 40-minute exercise regimen lost 10 percent of their visceral fat during the year that they were enrolled in the study.

<a href="http://ndb.nal.usda.gov/ndb/foods/show/2205?man=&lfacet=&count=&max=&qlookup=&offset=&sort=&format=Abridged&reportfmt=other&rptfrm=&ndbno=&nutrient1=&nutrient2=&nutrient3=&subset=&totCount=&measureby=&_action_show=Apply+Changes&Qv=1&Q4163=1.0&Q4164=1.0&Q4165=1.0&Q4166=.5" target="_blank">Half of an avocado</a> contains nearly 15 grams of fat total, and almost 10 of those are monounsaturated (2 grams are polyunsaturated). Try it <a href="http://www.huffingtonpost.com/2012/11/27/best-top-foods-heart-health_n_2200473.html" target="_blank">in place of mayo</a> on your next sandwich.